The preparation of isocyanates by the reaction of amines with phosgene in the gas phase has been reported as early as in the 1940's (see Siefken, Annalen 562, 108, 1949). The phosgenation reaction is a fast reaction process and usually carried out in a tube reactor. The gas-phase phosgenation requires a fast mixing rate and simultaneously avoid blockage of the reactor under high temperature as far as possible.
GB 1,165,831 describes a process for the gas-phase phosgenation of amines in vapor form with phosgene is carried out at a temperature of 300° C. in a tube reactor equipped with mechanical stirrer, which can prevent a build-up of polymer by-products on the wall of the tube. However, the reactor requires considerable safety precaution.
EP 0,289,840 adopts a cylindrical reactor without any moving parts, in which the reactants are reacted with one another while a turbulent flow is maintained in the reactor. Because the gas-phase phosgenation of aliphatic amines is a very fast reaction process, the reaction result is controlled by the mixing rate. However, due to back-mixing of the reactants, isocyanates may react with amines to form solid deposit in the reactor, which may obstruct the gas flow.
U.S. Pat. No. 4,847,408 adopts a reactor where gaseous reactants are mixed and react under a strong turbulent flow state. The reactor is measured 2.5 mm in inner diameter and 17.5 mm in length. Phosgene and gaseous hexamethylene diamine heated to 400° C. are continuously introduced into a cylindrical reaction chamber where they are mixed together to produce HDI. CN 1,396,152 improves the reactor described in U.S. Pat. No. 4,847,408 by converting the cylindrical reactor into a venturi reactor in shape. The venturi reactor has a converged mixing chamber and the cross section of the reaction chamber is enlarged abruptly along the flow direction of the gaseous reactants. Such a design can reduce back-mixing and the contact of the gaseous mixture with the inside wall of the reactor.
U.S. Pat. No. 6,082,891 describes the manufacture of H6TDI with a good reaction result using a microchannel mixer. This microchannel mixer is composed of a series of superposed lamellas with etched microchannels, thickness of the lamella being about 100 μm, and the channel size on the lamella being of several tens μm. Each layer of the lamellas can only transport one kind of gaseous reactants, phosgene and the amine vapor pass the lamellas alternately. The gaseous reactants emerging from the microchannel mixer are rapidly mixed, meanwhile a dispersive or turbulent flow is maintained. Because the dispersive effect is enhanced before mixing, this type of mixer obtains a good mixing result. However, due to the small size of the channel, there is a risk that a polymer produced at a high temperature may block the channel.
It can be seen from the above comparison that the phosgenation reaction of amines in the gas phase is a fast reaction process. Polymer by-products may be produced if the mixing of the gaseous reactants is not efficient enough. Thus a high mixing rate is desired. It can also be seen from the prior art that the key to obtaining a good reaction result is to adopt a reactor with an excellent mixing efficiency while avoiding the production of solid by-products.